The invention relates to particulate filter media and its use in particulate filters for either gas or liquid filtration, especially to high efficiency particulate air (HEPA) filters.
High efficiency particulate air HEPA filters are utilized in a number of different industries including the nuclear industry to prevent chemical contamination. For various applications the filter should operate at high temperatures and have hign strength. The filter should have high efficiency at high flow rates. The filter design should also be as compact as possible to lessen the impact on the overall system design.
Presently, there are no commercially available particulate filters which have high efficiency, low flow resistance, high strength and high temperature resistance. Currently available filters generally have two or three of these characteristics but not all four. For example, reinforced membrane filters have high efficiency, reasonable flow resistance and high strength, but are destroyed at temperatures above about 200.degree.-250.degree. C., depending upon the composition. The standard glass fiber media used in high efficiency particulate air (HEPA) filters has a high efficiency and low flow resistance, but has low strength and low temperature (below 300.degree. C.) resistance.
The high efficiency of the standard glass fiber HEPA filters results from the small size of the fibers utilized. However, adhesives used to bond the glass fibers together limit the temperature; conventional HEPA filters are not operative at temperatures over 300.degree. C. Even if the glass fibers were held together by sandwiching between high temperature resistance screens, the glass fibers have a relatively low melting point. For high temperature filtration, refractory fibers must be used; unfortunately they do not have sufficiently small fiber diameters for high-efficiency air cleaning. The efficiency of a filter mat made from high-temperature resistance non-conducting fibers can be significantly increased by conversion to an electric air filter in which particles are removed from the air electrostatically, but this requires more complex apparatus and the presence of a high voltage.
HEPA filters used to protect both workers and the environment from hazardous air-borne material originally was designed for applications having relatively low particle loadings and essentially ambient temperature and pressure conditions. This is why commercial HEPA filters are very satisfactory for routine applications; they are prone to failure at high temperatures, high pressures, and high humidities. HEPA filters used in the nuclear industry are very effective in removing radioactive particles when used under conditions of low temperature, low humidity, and low flow-rates, however, current HEPA filters have their greatest problem under accident conditions in which the filters may fail due to high temperature, high humidity, and overpressure conditions. Although accidents may be rare, a filter failure in these circumstances could have serious consequences.
One approach to the problem is to fabricate HEPA filters from a fibrous, stainless steel media. However, although stainless steel filters can withstand high temperature, pressure and humidity, they do not have efficiencies comparable to conventional HEPA filters. The problem is that the smallest stainless steel fiber diameter that can be produced is about 2 microns and in order to achieve efficiencies comparable to current HEPA filters, it is necessary to have fiber diameters of about 0.5 microns. Thus, sintered metal filters have only a 65% efficiency or less at its minimum. The primary reason for the use of glass micro-filters in HEPA filters is the availability of bulk fibers having diameters as small as 0.3 microns which are responsible for the high efficiency of HEPA filters. Unfortunately, these micro-fibers cannot be formed into a high strength filter media using conventional adhesives.
Composite materials have been used in an attempt to improve properties of filter media. A reinforced HEPA filter media of glass fibers on a supporting screen provides the desired strength but is still subject to high temperature failure because the binder holding the fibers together will be burned off. The fibers will then be loose and not retained on the supporting screen. An alternate approach of growing metal oxide whiskers on metallic screens, in which the small diameter whiskers provide high-efficiency, while the metal screen provides high strength, has been successful only for copper screens.
A variety of other filters are known in the art for various applications, as illustrated by the following U.S. patents. However, none of these provide a HEPA filter having high-efficiency, low-flow resistance, high strength and high-temperature resistance.
U.S. Pat. No. 2,994,577 to Silverman shows a copper fibrous filter coated with silver for removing iodine from gases; the silver coating prevents the copper from oxidizing and also participates in a chemical reaction with iodine.
U.S. Pat. 3,217,471 to Silverman describes a support structure for filter media that can withstand high pressure pulses; the support comprises a honeycomb structure in a flat screen in which the filter media rests. An absorbent material such as activated charcoal, silver plated silica gel and silver-plated copper turnings is disposed in a honeycomb matrix formed of aluminum, stainless steel or reinforced fiberglass.
U.S. Pat. No. 3,299,620 to Hollingworth describes a complex air cleaning system in which air is first cleaned by a series of particulate and gas filters, then passes through a water scrubber, a demister and a series of particulate and gas filters, and finally passes through a germicidal element. Commercially available filter media are used, such as coated spun glass, copper, aluminum or shredded steel.
U.S. Pat. No. 4,088,737 to Thomas et al. shows a silver exchanged zeolite filter. U.S. Pat. No. 4,004,971 to Freck et al. shows a graphite block filter element for removing cesium gas and particles from a nuclear reactor. U.S. Pat. No. 2,982,858 to Hoyer et al. describes an atomic particle generating device with a sintered glass element to separate the plasma from the accelerator portion of the generator. U.S. Pat. No. 1,970,700 to Kendall describes an apparatus for removing trace impurity gas from a gas stream; oxygen is removed from gas streams by chemical reaction with various metals heated to high temperature.